1. Field of the Invention
The present invention relates generally to LCD displays and their associated driving circuits and in particular to LCD display column driving circuits that use redundant circuits to compensate for any damaged columns or column data driving circuits.
2. Description of Related Art
An active matrix liquid crystal display device commonly comprises a plurality of picture elements, arranged in a matrix of rows and columns supported on a substrate. Each picture element consists of a switching transistor, a liquid crystal cell that can be modeled electrically as a capacitor and known as a pixel capacitor, and an optional storage capacitor. One of the electrodes of the pixel capacitor is on one side of the matrix display and a common electrode for all of the pixel capacitors is formed on the opposite side of the matrix display. The substrates are separated by a layer of electro-optic material. A pixel element including a liquid crystal cell and an optional storage capacitor are driven by a thin-film transistor (TFT) as a switching device. The transistor is used for transferring an input data signal to the corresponding pixel element.
The resolution of the picture is defined as the number of pixels in a display area. In a conventional, unscanned LCD display with X columns and Y rows, the display actually requires X+Y column and row driver leads. Therefore, it is clear, in the case of unscanned LCD displays, that the greater the number of pixels in a display area, the more input leads are required to connect external signals to the LCD display. One solution to the problem is to integrate a data driver circuit directly on the display substrate, which is typically of glass. This would reduce the number of input leads and external driving chips drastically. An integrated data driving circuit for LCD displays is described in commonly assigned co-pending application Ser. No. 971,721 filed Nov. 3, 1992, and entitled "Data Driving Circuit for LCD Display". The invention therein discloses a data driver circuit that uses a multiplexing arrangement to reduce the number of input data signal lines. Many times, in the manufacturing of such display devices, a damaged section of the data driver circuit, caused primarily by defects such as bad transistors, shorted lines, open lines, interlayer shorts and the like as well as open data lines inside of the display area, can lead to the corresponding data lines malfunctioning. In these cases, a redundant data driver circuit is usually required to compensate for these otherwise irreversible defects. As a result, the manufacturing yield is increased and the manufacturing cost is reduced.
It would be advantageous to provide a redundant data driving circuit on the substrate of an active matrix liquid crystal display device. In such case, damaged transistors and/or damaged lines in a column driver subcircuit could be replaced so as to form an acceptable display device.